In a traditional circuit-switched telecommunications network the voice stream, or, more generally, user media stream, is carried from an end terminal, such as a telephone, to a first point of switching, such as a Local Exchange, in fixed circuits or trunks, commonly termed an access network. The first point of switching forms the boundary of the so-called core network.
The circuit between the end terminal and the first point of switching may be an expensive commodity. The end terminal and the first point of switching may be separated by considerable distance. This is especially true in rural areas where it is not cost-effective to place a switch locally, thus requiring the transmission of media over long distances.
Where the cost of transmission is very high, such as with satellite links, compression schemes may be used on both voice and more general data streams.
In addition, there may be access network bandwidth aggregation or multiplex equipment between the terminal and the switch, so that only media streams with active connections occupy the available circuits Furthermore, there may be such schemes as Digital Communications Multiplex Equipment (DCME), which use voice activity detection to “share” the available circuits over active connections by carrying only active speech, and not silence periods. These schemes do not however alter the fundamental issue: there is a circuit media stream between the terminal and the switch whilst this terminal is active.
In many cases this access network transmission is fundamentally necessary. For example, if two end terminals which are connected by a call are in different locations, the media must be carried over the remote transmission link.
In some cases, however, the connection may be between two end terminal devices which share the same first point of switching. Both end connections will be connected by the same transmission link to the same switch. The media associated with the call is carried to the switch and then back again, occupying the transmission link for two connections, due to the first point of switching being remote.
In a packet-switched network, such as an IP network, using a so-called soft-switch architecture, the above problem may be eliminated. Although the low bandwidth call signalling from both terminals must be carried across the transmission link to the remote soft switch, the end terminals are controlled by the soft switch to send their higher bandwidth user media streams directly to the other end terminal. If both terminals are on the same end of the same transmission link, the media does not traverse the link, saving the possibly expensive link costs.
There are now several hybrid options between packet-switched and circuit-switched networks, one of which uses a packet-switched access network with a circuit-switched core network. This hybrid option utilises a gateway device between the two domains situated adjacent to the core network switch.
A reason for the move to a hybrid option is that in some circumstances it may be undesirable to modify the core network from circuit-switched to soft-switched. This may be due to the large investment in the capital cost, training, configuration, etc. of the switch, which is often a significant part of the overall costs of a telecommunications operator.
In such cases the operator would currently have to bare the costs of transmission for the local calls described above, even though the end terminals have the capability to route their media streams directly for local calls. This is because the switch is fundamentally the first point in the network to know the fact that the two access connections or legs of a call are indeed part of the same call, and thus may be directly connected.
An example of where the above situation exists is in a GSM (Global System for Mobile communications) network. Most GSM PLMN (Public Land Mobile Network) operators today have traditional circuit-switched Mobile Switching Centres (MSC), but some deploy IP-based Base Station Systems (BSS) (the access network for GSM), which are composed of a Base Station Controller (BSC) and Base Transceiver System (BTS). The BSC connects to the MSC using traditional E1 or Ti circuits and includes the gateway between the circuit-switched core network and the IP access network. The BSC and BTSs do not know if any of the connections they are carrying between the MSC and the BTSs (for onward connection to the Mobile Station (MS)) are part of the same call or not.